Epidural anaesthesia and high dose fentanyl (50 micrograms/kg) when used to supplement nitrous oxide and oxygen anaesthesia for a standard lower abdominal operation were associated with a mean fall in deep body temperature, as measured in the external auditory meatus, of 0.46 degrees C and 0.6 degrees C/hr respectively. This is significantly different from the mean values of 0.14 degrees C and 0.2 degrees C/hr which were recorded when moderate dose fentanyl (10 micrograms/kg) or 0.5% halothane were used to supplement anaesthesia. When mean skin temperature is combined with the core temperature to provide an estimate for total body heat, there is no difference between the groups. Redistribution of body heat occurs and this may be related to reduced adrenergic stimulation and altered regional blood flow. Where the facilities and environments are inadequate for the maintenance of normothermia, care should be taken in the choice of anaesthetic technique. In the recovery period mean body heat gain showed a wide scatter of results but those patients receiving epidural anaesthesia were slow to rewarm despite a high incidence of shivering in this group. The implications of this are discussed.